Despite the great promise of gene therapy, there remains to be solved the challenging problem of efficiently transferring and stably expressing transgenes in appropriate tissues. This problem has recently been termed the "vector void" (Hodgson, C P. Bio/Technol. 1995;13:222-225). After the description of DOTMA (N-[1-(2,3-dioleyloxy)propyl]-N,N,N-trimethylammonium chloride) (Felgner, P L, Gadek, T R, Holm, M, et al. Proc. Natl. Acad. Sci. USA. 1987;84:74137417), a plethora of cationic lipids have been synthesized. Basically, all the cationic lipids are amphipathic compounds that contain a hydrophobic domain, a spacer, and positively-charged amine(s). The cationic lipids are mixed with a fusogenic lipid such as DOPE (dioleoyl phosphatidyl ethanolamine) to form liposomes. The cationic liposomes are then mixed with plasmid DNA and the binary complex of the DNA and liposomes are then applied to cells in a tissue culture dish. The ease of mixing the plasmid DNA with the cationic liposome formulation, the ability of the cationic lipids to complex with DNA and the relative high levels of transfection efficiency has led to he increasing use of these formulations. However, these cationic lipid formulations can also be toxic to the cells in culture (Gao, X and Huang, L. Biochem. Biophys. Res. Com. 1991;179:280-285) (Leventis, R and Silvius, J R. Biochim. et Biophys. Acta 1990;1023:124-132). Although, LipofectAMINE can transfect many cell types more efficiently than other types of cationic lipid formulations (Harms, J S and Splitter, G A. Focus 1994;17:34-35), it also has greater toxicity (Hawley-Nelson, P, Ciccarone, V and Jessee, J. Focus 1993;15:73-79) (Ciccarone, V, Hawley-Nelson, P and Jessee, J. Focus 1993;15:80-83). If the transfection method is toxic to cells, then this would reduce its applicability for gene therapy. Cellular toxicity would also reduce its applicability to the study of genes since the cells would be in an altered state and it may be difficult to differentiate an effect of the transfection reagent from expression of the foreign gene.
Accordingly, there is a need for a means of transfecting cells with great efficiency and little toxicity. In addition, given the vector void for gene therapy, there is a need for new types of methods to transfer genes into mammalian cells.